The HMO Research Network CERT proposes to advance the nation's ability to assess comparative effectiveness of licensed therapies by creating tools that support the design, acceptability, and implementation of cluster randomized trials (CRTs), defined as prospective comparative studies that randomize groups of patients - typically at the level of medical practices or delivery systems. When these trials are appropriate and feasible, they have important advantages compared to conventional clinical trials, quasi-experimental studies, and observational methods. When performed in information-rich environments like the HMO Research Network, CRTs offer two major benefits: substantial efficiencies because of the ability to use automated data systems to acquire much of the needed baseline, exposure, and outcome data, and enhanced external validity due to the real world conditions of the trial. There is a substantial literature on cluster randomization and the HMO Research Network has considerable experience in using this method to test management strategies, but to date CRTs have seldom been used to study the comparative effectiveness of multiple treatments, such as two different drug regimens. The application of CRTs to questions of comparative effectiveness of therapeutics poses ethical, practical and logistical challenges. In order to progress in the use of this methodology, researchers must understand and address stakeholders' concerns in these areas. We propose to interview clinicians, health plan leadership, and purchasers to identify issues that must be addressed in order to perform CRTs in a manner that is acceptable to all parties, feasible and methodologically sound. We will also evaluate medical ethics and IRB issues that must be addressed, conducting an ethical analysis and developing ethical standards to inform researchers and health plans considering CRTs. We will pay special attention to the common situation in which the goal of a comparative effectiveness trial is to establish equivalence. We will ensure that our efforts are consonant with the needs of the Medicare Modernization Act by assessing the feasibility of CRTs for assessment of regimens for treatment of hypertension and depression - two conditions that are common causes of care by Medicare recipients. Products of this work will include white papers describing situations in which it may be desirable to conduct CRTs of therapeutic regimens, and guidance on developing such trials in a manner that is acceptable to the public, patients, the medical community, medical decision makers, and purchasers, and discussion of the ethical issues specific to these comparative effectiveness trials. Ultimately, this work will provide a foundation for the CERTs and other entities to conduct efficient, timely, valid and informative comparative trials of therapeutics. [unreadable] [unreadable] [unreadable]